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Contract No. DE-AC36-99-GO10337 

K. Emery, A. Anderberg, J. Kiehl, C. Mack, 

T. Moriarty, S. Rummel, and L. Ottoson 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-36527 

Trust But Verify: Procedures 

to Achieve Accurate Efficiency 

Measurements for All 

Photovoltaic Technologies 
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TRUST BUT VERIFY: PROCEDURES TO ACHIEVE ACCURATE 

EFFICIENCY MEASUREMENTS FOR ALL PHOTOVOLTAIC TECHNOLOGIES 

K. Emery, A. Anderberg, J. Kiehl, C. Mack, T. Moriarty, S. Rummel, and L. Ottoson 

National Renewable Energy Laboratory (NREL), Golden, CO 80401

ABSTRACT 

The measurement of the photovoltaic (PV) performance with respect to reference conditions requires measuring the performance with respect to a given tabular reference spectrum, junction temperature, and total irradiance. 

This paper discusses the procedures implemented by

NREL’s PV Cell and Module Performance Characterization Group to achieve the lowest practical uncertainty. 

This paper describes the process of trusting and verifying 

software, hardware, calibrations, procedures, and results. 

As an ISO 17025 accredited calibration facility, the quality

system that is in place is designed to assure customers 

that the results are valid within specified uncertainty limits 

and are traceable. The process of trusting claims but desiring an independent verification permeates the PV business and society.

INTRODUCTION 

The phrase “Trust but Verify” was popular during the 

Strategic Arm’s Limitation Talks in the 1980s, where the 

United States and Soviet Union trusted each other but 

required continuous unattended monitoring for compliance 

with treaty obligations. In the scientific community, the 

peer review process is critical to verify the quality of a 

manuscript. Errata and letters to the editor allow results to 

be challenged and defended. All laboratories must trust 

some other laboratory for at least part of their calibration 

traceability path for instruments that report a result. The 

level of trust that one has in a calibration depends on the 

laboratory’s stature as a national calibration facility (e.g., 

AIST, NIST, or PTB), an ISO 17025 accredited calibration 

laboratory, the original equipment manufacturer, or a national laboratory, such as NREL [1]. National standards 

laboratories and ISO 17025 accredited calibration laboratories have the highest stature because of the rigor in their 

procedures—a verified quality system. 

The same “trust but verify” axiom is applicable to the 

Photovoltaic Cell and Module Performance Characterization Group at NREL, where our primary function is verifying the performance of PV devices. The group must rely

on others for all of their instrumentation and detector radiometer calibrations. Most groups will trust that their 

equipment is in calibration or calibrated properly. ISO 

17025 requires that these calibrations be performed by a 

national standards facility such as NIST or an ISO 17025 

accredited laboratory. One of the key requirements is that 

these laboratories must demonstrate their proficiency

though periodic intercomparisons. NREL has participated 

in numerous formal [2-6] and informal intercomparisons 

over the years. This is an ongoing process where, at any

point, an intercomparison could reveal differences outside 

of estimated uncertainty limits. When this occurs, a detailed uncertainty analysis of both groups’ methods often 

reconciles differences.

CELL CALIBRATION 

Since the mid 1980s, there has been a consensus in 

the PV community for the reference spectrum, total irradiance, temperature, and area definition for flat-plate cells 

and modules. The journal Progress in Photovoltaics lists 

record PV cells and modules that have had independent 

efficiency verification by a PV calibration laboratory [7]. 

These tables give the community an alternative to trusting 

reported record efficiencies for a given technology. There 

has not been a consensus for rating concentrator cells or 

modules, and very limited domestic or international characterization standards exist. By default, the concentrator 

efficiency tables in ref. [7] use the ASTM direct reference 

spectrum [8]. A more realistic reference spectrum for concentrators is being considered [9]. This version is now an 

ASTM standard [10] and is being considered as a new IEC 

global and direct reference spectrum. It has been informally agreed among the PV calibration laboratories to 

switch to the new spectrum at the same time to minimize 

confusion. 

International PV standards groups have been unable 

to agree on an acceptable primary terrestrial and extraterrestrial calibration procedure. Each country felt that their 

methods were the most appropriate or best [5,6]. The 

terrestrial PV community attempted to reconcile this by

adopting the world photovoltaic scale, where the values 

are traceable to three or more laboratories whose primary

calibration value was within 1% [6,11]. In the context of 

this discussion, a primary reference cell is a cell that is not 

traceable to any other reference cell and is calibrated with 

SI traceable instrumentation. The extraterrestrial community decided to incorporate all methods in the standard 

with a distinction between high-altitude / space-based 

methods and ground-based or synthetic calibration methods [12]. 

The “trust but verify” axiom has proved critical in determining what is the correct reference spectrum. All published versions of the currently accepted terrestrial reference spectrum have typographical errors in their tables 

[13]. A more serious example occurred where the tabular 

AM0 reference spectrum [14] was different than the values 

published in the standard [15] that it reprinted with permis1
sion. This occurred because the shepherd for the standard had released a preliminary electronic version, but 

never sent a final electronic version of the tabular reference spectrum; the electronic values were never checked 

against the published values in ref. [15]. The wavelength 

values are all correct, the spectrum integrates to the correct value, and the shape is approximately correct. This 

mistake propagated to another group at NREL that also 

received the same early version and had it posted on their 

Web site from October 1999 to September 2004. In this 

case, everyone trusted the value, and only cursory verification occurred. In investigating this, it was discovered 

that the final values submitted to the ASTM committee for 

adoption and the actual published version differed by two 

digits out of 14,084 digits in the AM0 table [15]. At 260.5 

nm, the AM0 spectral irradiance that was submitted was 

85.51 Wm-2μm-1, whereas the published value was 88.51. 

And at 540.5 nm, the submitted value was 1769, whereas 

the published value was 1760 Wm-2μm-1 [15]. Which values are correct? 

Intercomparisons among terrestrial samples have 

shown an agreement of ±1% in the primary reference cell 

calibration value for Si [2,4-6]. Intercomparisons among 

AM0 calibration laboratories in the primary reference cell 

calibration value have shown agreement within ±2% for 

terrestrial (synthetic) calibrations and better than ±1% for 

aircraft- or balloon-based calibrations [16]. 

Differences in what is called the area is still a major 

source of difference, even though the definition has been 

standardized [17]. These differences can come from a 

variety of sources, with light piping, poor isolation, and 

small samples with irregular sides being the most common 

sources of differences. In some cases, an aperture may

be required to establish a well-defined area. Fortunately,

the national PV calibration laboratories and the Progress 

in Photovoltaics efficiency table authors are in agreement 

[7]. 

MEASUREMENT TEST BEDS 

All equipment producing a numerical result must be 

calibrated. The calibration traceability path for standardized cell and module measurements performed at NREL is 

shown in Fig. 1. Having well-calibrated equipment is not 

sufficient to ensure accurate measurements because calibrations can drift between the typical one-year calibration 

interval. The custom cell and module current-voltage (I-V) 

test beds at NREL attempt to use instrumentation that has 

to drift out of calibration 100 times or more to be at the 1% 

level in current or voltage. Other equipment may require 

periodic functional checks to ensure proper operation. 

Periodic performance verifications on a test sample are 

required to ensure that equipment is functioning properly. 

We recently had a case where a 4-wire 1-ohm resistor 

with a 0.01%/year stability, 15 ppm/°C temperature coefficient calibrated to better than 0.02% drifted by 1.35% after 

its annual recalibration. All of our custom test beds incorporate fuses in series with current sense resistors to prevent the current or power from ever approaching the 

manufacturer rated power. This resistor was fused to blow

at 2.3 W, or 7.5% of the manufacturer-specified resistor 

power rating. When notified of this out-of-tolerance condition, the calibration value was independently checked with 

a micro-ohm meter. The resistors are calibrated at their 

maximum and minimum current calculated by the software. As part of the quality assurance program, this nonconforming condition requires that all customers affected 

will be notified. The first problem is to determine—by examining previous proficiency checks, intercomparisons, 

and performance verifications—when in the past year the 

resistor actually drifted. To date, the results are inconclusive. As a result of lessons learned, all custom test beds 

now have software checks to ensure that the same current 

is measured within 0.1% for adjacent resistors. In our 

case, we use 100, 10, 1, 0.1, 0.01, and 0.001 precision 

resistors to cover a range on our various test beds from 

micro amps to 60 A. On one of our test beds, we have a 

current source, which is accurate to at least 0.1%, that 

serves as another check to ensure that this scenario will 

not occur again. It should be noted that commercial PV 

test systems typically have no internal checks to see if the 

current or voltage is significantly out of tolerance between 

calibrations. 

The group measures the cell and module I-V and 

quantum efficiency (QE) on a variety of commercial and 

custom test beds. The capabilities of these test beds are 

summarized in Table 1. Many of these test beds have 

overlapping voltage, current, and size ranges. This allows

the same sample to be tested on multiple test beds. This 

is critical to prevent a false sense of confidence in the 

data. Each test bed has its own error sources. The outdoor test bed has the best spatial uniformity, but the worst 

temperature control. Pulsed measurement systems do not 

have significant temperature-related errors, but errors related to bias rate or pre-measurement conditions can be 

significant. 

NIST

Primary

Reference cell

Secondary

Reference

cell

QE systems

Total Irradiance,

Cavity radiometer

Continuous solar

simulator - Modules

Pulsed Solar simulator

- Module

Natural sunlight -

Modules

NREL

Metrology

Spectral

Irradiance

World Radiometric Reference for

Total Irradiance (SI traceable)

Pulsed Solar simulator

- Concentrator cells

Figure 1. Traceability path for NREL Performance Characterization Group. 

Data acquisition and analysis software has been developed over the years. The process of verifying software 

written by someone else within the group is an essential 

part of the quality system. The software platform of LabView developed by National Instruments was chosen for 

all data acquisition and numerical analysis. The reason for 

this was that LabView, is transparent between Macintosh 

and PC operating systems, provides an excellent graphical 

user interface, allows easy testing sections of code, and 

enables descriptive variables and good structured programming practices to be followed. This is critical to be 

able to verify and approve software as required by an ISO 

17025 quality system. 

2
All test beds use standardized subprograms (or vi’s, 

using LabView terminology) for graphical output, spectral 

error computation, and curve-fitting portions of the I-V data 

to obtain the short-circuit current, Isc, open-circuit voltage, 

Voc, and maximum power point, Pmax, that are based on 

algorithms that work for nearly all cell and module technologies measured on all test beds. Most commercial 

algorithms fail for some cell types because they are not 

tested on all PV technologies by the manufacturer. At 

NREL, Isc is determined by performing a linear-regression 

fit to all I-V points that satisfy the constraint that all currents are within 4% of the current at 0 V and all voltages 

within 0.20 times the voltage at 0 A. Pmax is determined by

performing a polynomial fit of all I-V points that satisfy the 

constraints that the measured power be within 85% of the 

largest measured power and the voltage is within 80% of 

the voltage at the largest measured power. The polynomial that gives the best fit to the data up to a fifth order is 

used. The voltage at maximum power, Vm, is the real root 

of the derivative of the fit of the power versus voltage 

polynomial set equal to 0. This voltage is then substituted 

into a power versus voltage polynomial to obtain the Pmax. 

QUALITY SYSTEM 

ISO 17025 gives minimum guidelines for a quality

system [1]. The quality system must include a statement 

of management’s commitment to the quality of its testing 

and calibrations; objectives of the quality system; documentation of policies, systems, programs, procedures, and 

instructions to ensure the quality of the results; and roles 

and responsibilities of technical management for ensuring 

compliance with the quality system. Other requirements 

are backup personnel for all functions—including operators, software development, hardware development, calibration, and data review. Documentation of software, 

hardware, and calibration for all test beds is required. All 

software, forms, checklists, work instructions, and procedures require version control so that everyone is working 

off the same version. All data and calibrations must be 

reviewed by someone who was not involved in the process, but is trained to review the results. 

UNCERTAINTY ANALYSIS 

A comprehensive uncertainty analysis following the 

accepted guide for measurement uncertainty—

Measurement Uncertainty (GUM) of the International Bureau of Weights and Measures—was performed as part of 

NREL’s accreditation [18,19]. The GUM defines Type A 

uncertainty values as derived from statistical methods, and 

Type B sources as evaluated by "other means," such as 

scientific judgment, experience, specifications, comparisons, or calibration data. The GUM defines the concept of 

a "standard uncertainty" for each uncertainty type, which is 

an estimate of an "equivalent" standard deviation (of a 

specified distribution). The GUM replaces the historical 

factor of 2 with a "coverage factor," k (dependent on the 

known or assumed statistical distribution of uncertainties), 

and uncertainty U: 

U2

 = ∑(Type B) 2 + ∑(k · Type A)

2 (1) 

Uncertainty analysis allows the dominant sources of 

uncertainty to be identified. For PV applications, the reference device is typically a dominant error source. The 

spectral correction factor and area measurement can also 

be significant sources of error. Calibration functional 

checks, proficiency checks, and performance verifications 

are supposed to ensure that the maximum percent change 

in the calibrated value (e.g., efficiency, Isc, Voc) is below

some threshold. ISO 17025 accreditation requires that if 

calibrations are out of tolerance, then an analysis is required with customer notification if the out-of-tolerance 

condition is deemed significant. 

SUMMARY 

Obtaining ISO 17025 accreditation as a PV cell and 

module calibration laboratory is insufficient to ensure that 

the calibration results are valid within uncertainty estimates. The accredited calibration laboratory must verify

its results through internal and external intercomparisons. 

The group that requested the PV calibration should be 

able to verify that the cell did not change after calibration 

through dark I-V or other means, and by comparison with 

other standards calibrated previously. It is generally wise 

to have at least three stable calibrated reference cells that 

are recalibrated on an annual basis. 

For the health of the PV community, performance 

measurements on cells and modules performed by researchers or calibration laboratories must be periodically

verified by a third party. Groups that perform this on a 

regular basis establish a level of trust that future measurements will also be within tolerance or expected uncertainty limits. 

REFERENCES 

[1] ISO / IEC 17025, General Requirements for the Competence of Testing and Calibration Laboratories, 1999, 

International Standards Organization, Geneva, Switzerland. 

[2] H. Ossenbrink, R. Van Steenwinkel, and K. Krebs, 

”The Results of the 1984/1985 Round-Robin Calibration of 

Reference Solar Cells for the Summit Working Group on 

Technology, Growth and Employment,” Joint Research 

Center, ISPRA Establishment, ISPRA Italy, Tech Rep. 

EUR 10613 EN (April 1986). 

[3] K.A. Emery, C.R. Osterwald, L.L. Kazmerski, and 

R.E. Hart, “Calibration of Primary Terrestrial References 

Cells When Compared with Primary AM0 Reference 

Cells,” Proc. 8th European PSEC, 1988, pp. 64–68. 

[5] J. Metzdorf, et. al, “The Results of the PEP '87 

Round-Robin Calibration of Reference Cells and Modules, 

Final Report,” PTB Technical Report PTB-Opt-31, 

Braunschweig, Germany, November 1990, ISBN 3-89429-

067-6, see also 21st IEEE PVSC, 1990, p.95. 

[6] C.R. Osterwald, et. al,, “Results of the PEP ‘93 

Intercomparison of Reference Cell Calibrations and Newer 

Technology Performance Measurements,” 25th IEEE 

PVSC, 1996, pp. 1263-1266. see also March 1998, NREL 

Tech. Rep. NREL/TP-520-23477. 

[7] M.A. Green, K. Emery, D.L. King, S. Igari, and W.

Warta, “Solar Cell Efficiency Tables (version 24)” Progress 

3
[14] K.A. Emery, “Measurement and Characterization of 

Solar Cells and Modules,” Handbook of Photovoltaic 

Science and Engineering, Chap. 16, pp. 701-747, A. 

Luque and S. Hegedus, editors, John Wiley & Sons, W.

Sussex, UK, ISBN 0-471-49196-9, 2003. 

in Photovoltaics Research and Applications, 12, 2004, pp. 

365-372. 

[8] ASTM Standard G159, Standard Tables for References Solar Spectral Irradiance at Air Mass 1.5: Direct 

Normal and Hemispherical for a 37° Tilted Surface, Amer. 

Society for Testing Matls., West Conshocken PA, USA, 

1998. See also IEC standard 60904-3. 

[15] ASTM Standard E490, Standard Solar Constant and 

Zero Air Mass Solar Spectral Irradiance Tables, Amer. 

Society for Testing Matls., West Conshocken, PA, USA, 

2000.

[9] K. Emery, D. Myers, and S. Kurtz, “What is the 

Appropriate Reference Spectrum for Characterizing 

Concentrator Cells?,” 29th IEEE, 2002, pp. 840-843. [16] D. Brinker, et. al,, “Results from the 1st International 

AM0 Calibration Round Robin of Silicon and GaAs Solar 

Cells,” 3rd World Conference and Exhibition on 

Photovoltaic Solar Energy Conversion, 2003, paper 

3PC342. 

[10] ASTM Standard G173, Standard Tables for 

Reference Solar Spectral Irradiances: Direct Normal and 

Hemispherical on 37° Tilted Surface, Amer. Society for 

Testing Matls., West Conshocken PA, USA,2003. 

[17] K. Emery and H. Field, “Artificial Enhancements and 

Reductions in the PV Efficiency,” Proc. 24th IEEE 

Photovoltaic Specialists Conf., Waikoloa, HI, Dec.5-9, 

1994, pp. 1833-1838, IEEE, New York, 1994. 

[11] C.R. Osterwald, et. al, “The World Photovoltaic Scale: 

An International Reference Cell Calibration Program,” 

Progress in PV, 7, pp. 287-297, 1999. 

[12] ISO/DIS 15387, Space Systems – Space Solar Cells 

– Requirement, Measurements and Calibration 

Procedures, 2003. International Standards Organization, 

Geneva, Switzerland. 

[18] American Association for Laboratory Accreditation 

certificate A2LA 2236.01 for Secondary Reference Cell 

Calibration, Web site: www.a2la.org, Frederick, MD. 

[19] Procedure ISO GUM “International Organization for 

Standardization, Guide to the Expression of Uncertainty in 

Measurement,” ISO: Geneva, 1995, ISBN 92-67-10188-9. 

[13] C.A. Gueymard, D. Myers, and K. Emery, "Proposed 

Reference Irradiance Spectra for Solar Energy Systems 

Testing," Solar Energy, 73, pp. 443-467, 2002. 

Table 1. Instrumentation for Performance Measurements in the PV Cell and Module Measurement Group at NREL 

System Typical Applications Special Features Light Source Testbed Features

X25 1-sun cell & spectrally adjustable, Spectrolab X25 30 cm x 30 cm, 

 small modules user-controlled filtered 3-kW Xe 5°-80°C, 

bias conditions 0.1 to 20 suns ±0.5 mV to ±50 V 

ISO17025 ±10 pA to ±16 A 

LACSS 1-sun modules user-controlled Spectrolab X200 152 cm x 122 cm 

bias conditions filtered 25-kW Xe ±0.5 mV to ±300 V 

 0.1 to 20 suns ±1 μA to ±60 A 

Continuous concentrator cells spectrally adjustable, Xe lamp area ~ 1-cm diameter 

3-kW tungsten tungsten lamp area 5 cm x 10 cm 

user-controlled 1 to 200 suns 5°-80°C 

bias conditions ±0.1 mV to ±10 V 

±1 μA to ±10 A 

HIPSS concentrator cells spectrally adjustable, 2 Xe flash lamps 2 cm x 20 cm 

minimal heating, 30 cm long 0.1 mV to 100 V 

5°-80°C 1 to 2000 suns 500 μA to 50 A 

2 lamp housings with and 

 without mirrors to focus the 

light from the lamps 

SOMS outdoor flat-plate 2 -axis manual tracking sunlight 200 x 300 cm 

& concentrator meteorological parameters ±0.5 mV to ±300 V 

modules spectral irradiance measured ±1 μA to ±60 A 

user-controlled bias conditions 

Spire 240A 1-sun modules commercial system Spire 240A 61 cm x 122 cm 

25°C (20°-60°C possible) 1 Xe flash lamp 0.1 mV to 100 V 

0.1 to 1.2 suns 0.5 mA to 20 A 

Filter QE 1-sun cells, modules light, voltage bias, ISO17025 1-kW Xe lamp 298–1800 nm 

Grating QE Absolute cell QE voltage and light bias Xe or W 400–2500 nm 

Area X-Y stage 16 cm x 16 cm, ±1 μm 

Primary reference cell WPVS cal. lab, ISO17025 direct-beam sunlight 4 cells at a time 

4
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Trust But Verify: Procedures to Achieve Accurate Efficiency 

Measurements for All Photovoltaic Technologies 

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K. Emery, A. Anderberg, J. Kiehl, C. Mack, T. Moriarty, S. Rummel, 

and L. Ottoson 

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14. ABSTRACT (Maximum 200 Words)

The measurement of the photovoltaic (PV) performance with respect to reference conditions requires measuring the 

performance with respect to a given tabular reference spectrum, junction temperature, and total irradiance. This 

paper discusses the procedures implemented by NREL’s PV Cell and Module Performance Characterization Group 

to achieve the lowest practical uncertainty. This paper describes the process of trusting and verifying software, 

hardware, calibrations, procedures, and results. As an ISO 17025 accredited calibration facility, the quality system 

that is in place is designed to assure customers that the results are valid within specified uncertainty limits and are 

traceable. The process of trusting claims but desiring an independent verification permeates the PV business and 

society. 

15. SUBJECT TERMS

PV; efficiency measurements; performance; tabular reference spectrum; junction temperature; total irradiance; 

trusting and verifying software; 

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